Transparent polyester film with high oxygen barrier and additional functionality, its use and process for its production

ABSTRACT

The application discloses a transparent, biaxially oriented polyester film having a base layer at least 80% by weight of which is composed of a thermoplastic polyester, and having at least one outer layer, wherein the outer layer is composed of a polymer, or of a mixture of polymers, which comprises at least 40% by weight of ethylene 2,6-naphthalate units and up to 40% by weight of ethylene terephthalate units and/or units from cycloaliphatic or aromatic diols and/or dicarboxylic acids, with the proviso that the T g 2 value of the polyester film is above the T g 2 value of the base layer but below the T g 2 value of the outer layer, and at least one film surface has a surface tension of from 35 to 65 mN/m and/or has been provided with a functional coating of thickness from 5 to 100 nm. The film has low atmospheric oxygen transmission. It is particularly suitable for packaging uses, specifically for packaging foods and other consumable items, and for laminating, metallizing and printing.

This application is a continuation of application Ser. No. 09/274,781,filed on Mar. 24, 1999, now abandoned, the contents of which areincorporated by reference herein. This application also claims thebenefit of priority under 35 U.S.C. §119(a) to German patent applicationno. 198 13 267.0, filed on Mar. 25, 1998.

The invention relates to a transparent, biaxially oriented polyesterfilm having a base layer at least 80% by weight of which is composed ofa thermoplastic polyester, and having at least one outer layer, where atleast one film surface has been provided with an additionalfunctionality. The invention furthermore relates to the use of the filmand to a process for its production.

BACKGROUND OF THE INVENTION

Food and drink packaging frequently demands a high barrier effect withrespect to gases, water vapor and flavors. For this reason, use isusually made of polypropylene films which have been metallized or havebeen coated with polyvinylidene chloride (PVDC). However, metallizedpolypropylene films are not transparent and are therefore not used incases where the view of the contents is likely to have added promotionaleffect. Although films coated with PVDC are transparent, the coating,like the metallizing, takes place in a second operation which makes thepackaging markedly more expensive. Ethylene-vinyl alcohol copolymers(EVOH) likewise exhibit a strong barrier effect. However, films modifiedwith EVOH are particularly highly moisture-sensitive, and this limitstheir range of applications. In addition, because of their poormechanical properties they have relatively high thickness or have to belaminated with other materials at high cost, and they are also difficultto dispose of after use. Some raw materials, furthermore, have not beenapproved by the authorities or are unsuitable for producing food anddrink packaging.

It is therefore an object of the present invention to provide atransparent, biaxially oriented polyester film which has a high oxygenbarrier and at least one additional functionality, and which can beproduced simply and cost-effectively, has the good physical propertiesof the known films and does not give rise to disposal problems.

DESCRIPTION OF THE INVENTION

The object is achieved by means of a biaxially oriented polyester filmhaving a base layer at least 80% by weight of which is composed of (atleast) a thermoplastic polyester, and having at least one outer layer,wherein the film has (an) outer layer(s) composed of a polymer or amixture of polymers which comprises at least 40% by weight of ethylene2,6-naphthalate units and up to 40% by weight of ethylene terephthalateunits and/or up to 60% of units from aliphatic, includingcycloaliphatic, or aromatic diols and/or dicarboxylic acids, with theproviso that the glass transition temperature (T_(g)2 value) of thepolyester film is above the T_(g)2 value of the base layer but below theT_(g)2 value of the outer layer, and at least one film surface has asurface tension of from 35 to 65 mN/m and/or has been provided with afunctional coating of thickness from 5 to 100 nm. The novel filmgenerally has an oxygen permeability of less than 80 cm³/(m² bar d),preferably less than 75 cm³/(m² bar d), particularly preferably lessthan 70 cm³/(m² bar d).

Preference is given to a polyester film in which the polymers of theouter layer comprise at least 65% by weight of ethylene 2,6-naphthalateunits and up to 35% by weight of ethylene terephthalate units. Amongthese, particular preference is then given to a polyester film of thetype in which the polymers of the outer layer comprise at least 70% byweight of ethylene 2,6naphthalate units and up to 30% by weight ofethylene terephthalate units. The outer layer may, however, also becomposed completely of ethylene 2,6-naphthalate polymers.

Examples of suitable aliphatic diols are diethylene glycol, triethyleneglycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH, where n is aninteger from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol,1,5-pentanediol and 1,6-hexanediol), or branched aliphatic glycolshaving up to 6 carbon atoms, and cycloaliphatic diols having one or morerings and if desired containing heteroatoms. Among the cycloaliphaticdiols, mention may be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of suitable aromatic diols are those ofthe formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—,—O—, —S— or —SO₂—. Besides these, bisphenols of the formulaHO—C₆H₄—C₆H₄—OH are also very suitable.

Preferred aromatic dicarboxylic acids are benzenedicarboxylic acids,naphthalenedicarboxylic acids (for example naphthalene-1,4- or-1,6dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids, mention may be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C₃-C₁₉-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

The present invention also provides a process for producing this film.It encompasses

-   a) producing a film from base and outer layer(s) by coextrusion,-   b) biaxial orientation of the film and-   c) heat-setting of the oriented film, and-   d) functionalizing at least one film surface before, during or after    steps b and/or c.

To produce the outer layer, it is expedient to feed granules ofpolyethylene terephthalate and polyethylene 2,6-naphthalate directly tothe extruder in the desired mixing ratio. At about 300° C. and with aresidence time of about 5 min, the two materials can be melted and canbe extruded. Under these conditions, transesterification reactions canoccur in the extruder and during these copolymers are formed from thehomopolymers.

The polymers for the base layer are expediently fed in via anotherextruder. Any foreign bodies or contamination which may be present canbe filtered off from the polymer melt before extrusion. The melts arethen extruded through a coextrusion die to give flat melt films and arelayered one upon the other. The coextruded film is then drawn off andsolidified with the aid of a chill roll and other rolls if desired.

The biaxial orientation is generally carried out sequentially orsimultaneously. For the sequential stretching, it is preferable toorient firstly in a longitudinal direction (i.e. in the machinedirection) and then in a transverse direction (i.e. perpendicularly tothe machine direction). This causes an orientation of the molecularchains. The orientation in a longitudinal direction may be carried outwith the aid of two rolls running at different speeds corresponding tothe stretching ratio to be achieved. For the transverse orientation, useis generally made of an appropriate tenter frame. For the simultaneousstretching, the film is stretched in a tenter frame simultaneously in alongitudinal direction and in a transverse direction.

The temperature at which the orientation is carried out can vary over arelatively wide range and depends on the properties desired in the film.In general, the longitudinal stretching is carried out at from 80 to130° C., and the transverse stretching at from 90 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1. If desired, the transverse orientation may be followed by anotherlongitudinal orientation and even a further transverse orientation.

During the subsequent heat-setting, the film is held for from 0.1 to 10s at a temperature of from 150 to 250° C. The film is then wound up in aconventional manner.

A great advantage of this process is that it is possible to feed theextruder with granules, which do not block the machine.

The base layer of the film is preferably composed to an extent of atleast 90% by weight of the thermoplastic polyester. Polyesters suitablefor this are those made from ethylene glycol and terephthalic acid(=polyethylene terephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN),from 1 ,4-bishydroxymethylcyclo-hexane and terephthalic acid(=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters whichare composed to an extent of at least 90 mol %, preferably at least 95mol %, of ethylene glycol units and terephthalic acid units or ofethylene glycol units and naphthalene-2,6-dicarboxylic acid units. Theremaining monomer units are derived from other diols and/or dicarboxylicacids. Examples of suitable diol comonomers are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6, branched aliphatic glycols having upto 6 carbon atoms, aromatic diols of the formula HO—C₆H₄—X—C₆H₄—OH whereX is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—, or bisphenols ofthe formula HO—C₆H₄—C₆H₄—OH.

The dicarboxylic acid comonomer units are preferably derived frombenzenedicarboxylic acids, naphthalenedicarboxylic acids,biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl4,4′-dicarboxylic acid), cyclohexane-dicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid), stilbene-x,x′-dicarboxylicacid or C₁-C₁₆-alkane-dicarboxylic acids, where the alkane moiety may bestraight-chain or branched.

The polyesters may be prepared by the transesterification process. Thestarting materials for this are dicarboxylic esters and diols, which arereacted using the customary transesterification catalysts, such as saltsof zinc, of calcium, of lithium and of manganese. The intermediates arethen polycondensed in the presence of widely used polycondensationcatalysts, such as antimony trioxide or titanium salts. The preparationmay be carried out just as successfully by the direct esterificationprocess in the presence of polycondensation catalysts, starting directlyfrom the dicarboxylic acids and the diols.

For processing the polymers, it has proven useful to select the polymersfor the base layer and the outer layer(s) in such a way that theviscosities of the respective polymer melts do not differ excessively.Otherwise it is likely that there will be flow disturbances or streakson the finished film. To describe the viscosity ranges of the two melts,use is made of a modified solution viscosity (SV). The solutionviscosity is a measure of the molecular weight of the respective polymerand correlates with the melt viscosity. The chemical make-up of thepolymer used may result in other correlations. For commerciallyavailable polyethylene terephthalates which are suitable for producingbiaxially oriented films, the SVs are in the range from 600 to 1000. Toensure satisfactory film quality, the SV of the copolymers for the outerlayer should be in the range from 300 to 900, preferably between 400 and800, in particular between 500 and 700. If desired, a solid phasecondensation may be carried out on the respective granules in order toadjust the SVs of the materials as necessary. It is a general rule thatthe melt viscosities of the polymer melts for base and outer layer(s)should differ by not more than a factor of 5, preferably not more than afactor of from 2 to 3.

The polymers for the outer layer may be prepared in three differentways:

-   a) In copolycondensation, terephthalic acid and    naphthalene-2,6-dicarboxylic acid are placed in a reactor together    with ethylene glycol, and polycondensed to give a polyester, using    the customary catalysts and stabilizers. The terephthalate and    naphthalate units are then randomly distributed in the polyester.-   b) Polyethylene terephthalate (PET) and polyethylene 2,6-naphthalate    (PEN), in the desired ratio, are melted together and mixed, either    in a reactor or preferably in a melt kneader (twin-screw kneader) or    in an extruder. Immediately after the melting, transesterification    reactions between the polyesters begin. Initially, block copolymers    are obtained, but as reaction time increases—depending on the    temperature and mixing action of the agitator—the blocks become    smaller, and long reaction times give a random copolymer. However,    it is not necessary and also not always advantageous to wait until a    random distribution has been achieved, since the desired properties    are also obtained with a block copolymer. The resultant copolymer is    then extruded from a die and granulated.-   c) PET and PEN are mixed as granules in the desired ratio, and the    mixture is fed to the extruder for the outer layer. Here, the    transesterification to give the copolymer takes place directly    during the production of the film. This process has the advantage of    being very cost-effective, and generally gives block copolymers, the    block length being dependent on the extrusion temperature, the    mixing action of the extruder and the residence time in the melt.

In a preferred embodiment of the invention, from 0.1 to 20% by weight ofthe polymers of the base layer are identical with those of the outerlayer. These are either directly admixed with the base layer duringextrusion or are in any case present in the film due to addition ofregenerated material. The proportion of these copolymers in the baselayer is selected in such a way that the base layer has a partiallycrystalline character.

In another embodiment, the film encompasses, on the side facing awayfrom the outer layer, another outer layer of polyethylene terephthalate,and this layer comprises pigments.

The novel film exhibits a surprisingly high oxygen barrier. If, incontrast, the polymers used for the outer layer(s) comprise(s) less than40% by weight of ethylene 2,6-naphthalate units and more than 40% byweight of ethylene terephthalate units then, although the film in somecases has somewhat lower oxygen transmission than a standard polyesterfilm (composed to an extent of 100% by weight of polyethyleneterephthalate), the transmission is still much too high. It has evenbeen found that the oxygen barrier is poorer than in a standardpolyester film if the outer layer comprises from 30 to 40% by weight ofethylene 2,6-naphthalate units and from 60 to 70% by weight of ethyleneterephthalate units. However, even underthese circumstances there may beadvantage in a film having an outer layer which comprises at least 5%,preferably between 5 and 40%, by weight of ethylene 2,6-naphthalateunits and more than 40% by weight of ethylene terephthalate units, ifthe oxygen barrier does not play a decisive part in the applicationconcerned.

In the novel films, moreover, the glass transition temperature T_(g) ofthe (co)polymer or of the (co)polymers of the outer layer(s) differsfrom the prior art in being higher than the glass transition temperatureT_(g) of the polymers of the base layer. The glass transitiontemperature of the (co)polymers used for the outer layer(s) ispreferably in the range from 80 to 120° C. In the DSC (differentialscanning calorimetry) determination of the glass transitiontemperatures, the transitions of the two layers cannot bedifferentiated.

Glass transitions which are determined on biaxially oriented, heat-setfilms in the first heating procedure (termed T_(g)1 below) are, due tocrystallinity and also to molecular stresses in the amorphous fractionof the specimens, relatively small in size, distributed over a widetemperature range, and shifted to higher temperatures. Because oforientation effects in particular, they are not suitable forcharacterizing a polymer. The resolution of DSC analyzers is ofteninsufficient to detect the glass transitions in the first heatingprocedure (T_(g)1) of the individual layers of the novel film, thetransitions being “blurred” and small, due to orientation andcrystallinity. If the specimens are melted and then rapidly cooled againto below their glass transition temperature (quenched), the orientationeffects are eliminated. On renewed heating, glass transitions(designated T_(g)2 here) are then measured which have a greaterintensity and are characteristic of the respective polymers. However,even here it is not possible to differentiate the glass transitions ofthe individual layers, since the layers mix on melting and thepolyesters present therein enter into transesterification reactions withone another. It is fully sufficient, however, to compare the T_(g)2 ofthe entire coextruded films with the T_(g)2 of the polymer used for thebase layer. In known films the T_(g)2 value of the base layer is higherthan that of the coextruded film, whereas the T_(g)2 value of the outerlayer is lower than that of the base layer and also than that of thecoextruded film. Exactly the opposite of this applies for the novelfilm. Here, the T_(g)2 value of the coextruded film is higher than thatof the base layer but lower than the T_(g)2 value of the outer layer.

The base layer and the outer layer(s) may, in addition, comprisecustomary additives, such as stabilizers and antiblocking agents. Theyare expediently added to the polymer or to the polymer mixture beforemelting takes place. Examples of stabilizers are phosphorus compounds,such as phosphoric acid and phosphoric esters. Typical antiblockingagents (also termed pigments in this context) are inorganic and/ororganic particles, for example calcium carbonate, amorphous silica,talc, magnesium carbonate, barium carbonate, calcium sulfate, bariumsulfate, lithium phosphate, calcium phosphate, magnesium phosphate,aluminum oxide, LiF, the calcium, barium, zinc and manganese salts ofthe dicarboxylic acids used, carbon black, titanium dioxide, kaolin,crosslinked polystyrene particles or crosslinked acrylate particles.

The additives selected may also be mixtures of two or more differentantiblocking agents or mixtures of antiblocking agents of the samemake-up but of different particle size. The particles may be added tothe individual layers in the customary concentrations, e.g. as glycolicdispersion during the polycondensation or via masterbatches duringextrusion. Pigment concentrations of from 0.0001 to 5% by weight haveproven particularly suitable. A detailed description of the antiblockingagents is found, for example, in EP-A-0 602 964.

According to the invention, at least one film surface has been treatedin such a way that its surface tension is in the range from 35 to 65mN/m, preferably from 40 to 60 mN/m, in particular from 45 to 55 mN/m.This is achieved by a corona and/or flame treatment, which usuallyfollows the heat-setting of the film. The treatment can also take placeat other points in the film production process, e.g. before or after thelongitudinal stretching. Alternatively or additionally to the surfacetreatment described above, at least one side of the film may be coated,in such a way that the coating on the finished film has a thickness offrom 5 to 100 nm, preferably from 20 to 70 nm, in particular from 30 to50 nm. The coating is preferably applied in-line, i.e. during the filmproduction process, and usefully before the transverse stretching. It isparticularly preferable for the application process to be reversegravure-roll coating, which allows extremely homogeneous application ofthe coatings in the thicknesses mentioned. The coatings are preferablyapplied as solutions, suspensions or dispersions, particularlypreferably as aqueous solution, suspension or dispersion. The coatingsmentioned give the film surface an additional function, e.g. make thefilm sealable, printable, metallizable, sterilizable or antistatic, orimprove, for example, the flavor barrier, or permit adhesion tomaterials which would not otherwise adhere to the film surface (e.g.photographic emulsions). Examples of substances/compositions which giveadditional functionality are:

Acrylates, as described, for example, in WO 94/13476, ethylvinylalcohols, PVDC, water glass (Na₂SiO₄), hydrophilic polyesters(5-Na-sulfoisophthalic-acid-containing PET/IPA polyesters, as described,for example in EP-A-0,144,878, U.S. Pat. No. 4,252,885 orEP-A-0,296,620), vinyl acetates, as described, for example, in WO94/13481, polyvinyl acetates, polyurethanes, the alkali metal oralkaline-earth metal salts of C₁₀-C₁₈ fatty acids, butadiene copolymerswith acrylonitrile or methyl methacrylate, methacrylic acid, acrylicacid or esters thereof.

The substances/compositions mentioned are applied in the form of dilutesolution, emulsion or dispersion, preferably as aqueous solution,emulsion or dispersion, to one or both film surfaces, and then thesolvent is evaporated. If the coatings are applied in-line beforetransverse stretching, the heat treatment in the transverse stretchingand subsequent heat-setting is usually sufficient to evaporate thesolvent and to dry the coating. The dried coatings then have thicknessesof from 5 to 100 nm, preferably from 20 to 70 nm, in particular from 30to 50 nm.

The novel polyester film preferably also comprises a second outer layer.The structure, thickness and make-up of a second outer layer may beselected independently of the outer layer already present, and thesecond outer layer may likewise comprise the abovementioned polymers orpolymer mixtures, but these do not necessarily have to be identical withthose of the first outer layer. The second outer layer may also compriseother commonly used outer layer polymers.

Between the base layer and the outer layer(s), there may also be anintermediate layer if desired. It may be composed of the polymersdescribed for the base layers. In a particularly preferred embodiment,it is composed of the polyester used for the base layer. It may alsocomprise the customary additives described. The thickness of theintermediate layer is generally greater than 0.3 μm and is preferably inthe range from 0.5 to 15 μm, in particular from 1.0 to 10 μm.

The thickness of the outer layer(s) is generally greater than 0.1 μm andis preferably in the range from 0.2 to 6.0 μm, more preferably in therange from 0.3 to 5.5 μm, in particular from 0.3 to 5.0 μm. It ispossible for the outer layers to have identical or differentthicknesses.

The total thickness of the novel polyester film may vary within widelimits and depends on the application envisaged. It is preferably from 4to 100 μm, in particular from 5 to 50 μm, preferably from 6 to 30 μm,the base layer preferably presenting a proportion of from about 40 to90% of the total thickness.

A further advantage is that the production costs of the novel film areonly insignificantly greater than those of a film made from standardpolyester raw materials. The other properties of the novel film whichare relevant to processing and use remain essentially unchanged or areeven improved. In addition, it has been ensured that regeneratedmaterial can be used in a proportion of up to 50% by weight, preferablyfrom 10 to 50% by weight, based on the total weight of the film in eachcase, in the production of the film without significant adverse effecton its physical properties.

The film has excellent suitability for packaging foodstuffs and otherconsumable items and, depending on the functionality of one or both ofits surfaces, as photographic film, graphic film, laminatable film,metalizable film or printable film. For some of these uses, the filmsare usually metallized or are ceramically coated (e.g. with SiO_(x),Al_(x)O_(y), Na₂SiO₄, etc.).

The following methods were used to characterize the raw materials andthe films:

The oxygen barrier was measured using a Mocon Modern Controls (USA)OX-TRAN 2/20 in accordance with DIN 53 380, Part 3.

The SV (solution viscosity) was determined by dissolving a specimen ofpolyester in a solvent (dichloroacetic acid) (1% strength by weightsolution). The viscosity of this solution and that of the pure solventwere measured in an Ubbelohde viscometer. The quotient (relativeviscosity η_(rel)) was determined from the two values, 1.000 wassubtracted from this, and the value multiplied by 1000. The result wasthe SV.

The coefficient of friction was determined according to DIN 53 375, 14days after production.

The surface tension was determined using the “ink method” (DIN 53 364).

The haze of the film was measured in accordance with ASTM-D 1003-52. TheHölz haze was determined by a method based on ASTM-D 1003-52, but inorder to utilize the most effective measurement range, measurements weremade on four pieces of film laid one on top of the other, and a 1° slitdiaphragm was used instead of a 4° pinhole.

Gloss was determined in accordance with DIN 67 530. The reflectance wasmeasured as a optical characteristic value for a film surface. Based onthe standards ASTM-D 523-78 and ISO 2813, the angle of incidence was setat 20° or 60°. A beam of light hits the flat test surface at the setangle of incidence and is reflected and/or scattered thereby. Aproportional electrical variable is displayed representing light rayshitting the photoelectronic detector. The value measured isdimensionless and must be stated together with the-angle of incidence.

The glass transition temperatures T_(g)1 and T_(g)2 were determinedusing film specimens with the aid of DSC (differential scanningcalorimetry). Use was made of a DuPont DSC 1090. The heating rate was 20K/min and the specimen weight was about 12 mg. In the first heatingprocedure, the glass transition T_(g)1 was determined. Many of thespecimens showed an enthalpy relaxation (a peak) at the beginning of thestep-like glass transition. The temperature taken as T_(g)1 was that atwhich the step-like change in heat capacity—without reference to thepeak-shaped enthalpy relaxation—achieved half of its height in the firstheating procedure. In all cases, there was only a single glasstransition stage in the thermogram in the first heating procedure. It ispossible that the peak-shaped enthalpy relaxations obscured the finestructure of the step, or that the dissolution of the device was notadequate to separate the small, “blurred” transitions of oriented,crystalline specimens. To eliminate their heat history, the specimenswere held at 300° C. for 5 minutes after the heating procedure, and thenquenched with liquid nitrogen. The temperature for the glass transitionT_(g)2 was taken as the temperature at which the transition reached halfof its height in the thermogram for the second heating procedure.

The following examples illustrate the invention. The products used(trademarks and manufacturers) are given only once in each case, andthen relate to the examples which follow.

EXAMPLE 1

The polymer for the outer layer was prepared by copolycondensation. Forthis, dimethyl terephthalate and 2,6-dimethyl naphthalenedicarboxylatewere mixed in a reactor in a molar ratio of 0.54:1.00 (corresponding toa make-up of 30% by weight of ethylene terephthalate units and 70% byweight of ethylene 2,6-naphthalate units in the final copolymer), andthen mixed with ethylene glycol and, as catalyst, 300 ppm of manganeseacetate. The transesterification was carried out with stirring at from160 to 250° C., at atmospheric pressure, and the methanol obtainedduring this process was distilled off. An equimolar amount of phosphoricacid, as stabilizer, and 400 ppm of antimony trioxide, as catalyst, werethen added. The polycondensation was carried out with stirring at 280°C. and a pressure of less than 1 mbar. The molecular weight achievedcould be determined by measuring the torque on the stirrer. After thereaction, nitrogen pressure was used to discharge the melt from thereactor, and it was then pelletized.

EXAMPLE 2

Commercially available polyethylene terephthalate pellets andpolyethylene 2,6-naphthalate pellets were used. In each case, thepellets were crystallized and dried for about 4 h at a temperature ofabout 160° C. The two materials in a ratio of 30:70 (30% by weight ofpolyethylene terephthalate and 70% by weight of polyethylene2,6-naphthalate) were then placed in a mixer, where they werehomogenized by stirring. The mixture was then passed to a twin-screwcompounder (ZSK from Werner and Pfleiderer, Stuttgart), where it wasextruded at a temperature of about 300° C. and with a residence time ofabout 3 min. The melt was extruded and chipped. A copolymer was producedin the extrusion by reaction between the polyethylene terephthalate andpolyethylene 2,6-naphthalate.

EXAMPLE 3

Example 2 was repeated, but, for production of the film, chips ofpolyethylene terephthalate and of polyethylene 2,6-naphthalate were fedin a mixing ratio of 3:7 directly to the single-screw extruder, wherethe two materials were extruded at about 300° C. The melt was filteredand extruded through a coextrusion die to give a flat film, and laid asouter layer onto the base layer. The coextruded film was discharged overthe die lip and solidified on a chill roll. The residence time of thetwo polymers in the extrusion was about 5 min. Further processing stepswere as given above. Here, too, the copolymer was produced in theextrusion under the conditions given.

EXAMPLE 4

Chips of polyethylene terephthalate were dried at 160° C. to a residualmoisture of less than 50 ppm and fed to the extruder for the base layer.Besides this, chips of polyethylene terephthalate and polyethylene2,6naphthalate (in a weight ratio of 3:7) were likewise dried at 160° C.to a residual moisture of 50 ppm and fed to the two extruders for theouter layers. The extruder conditions for the outer layers were as inExample 3.

A transparent three-layer film of symmetrical structure and an overallthickness of 12 μm was then produced by coextrusion followed by stepwiseorientation in the longitudinal and transverse directions. Each of theouter layers has a thickness of 2.0 μm.

Base layer: 95% by weight of polyethylene terephthalate (RT 49 fromHoechst AG) having an SV of 800 and  5% by weight of masterbatch madefrom 99% by weight of polyethylene terephthalate and 1.0% by weight ofsilica particles (SYLOBLOC ® 44 H from Grace) having an average particlesize of 4.5 μm. Outer layers: 70% by weight of polyethylene2,6-naphthalate (POLYCLEAR ® N 100 prepolymer from Hoechst AG) having anSV of 800, 20% by weight of polyethylene terephthalate having an SV of800 and 10% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles havingan average particle size of 2.0 μm.

The individual steps were:

Extrusion Temperatures: Outer layer: 300° C. Base layer: 300° C.Temperature of the take-off 30° C. roll: Die gap width: 1 mm Temperatureof the take-off 30° C. roll: Longitudinal Temperature: 85-135° C.stretching Longitudinal stretching ratio: 4.0:1 Transverse Temperature:85-135° C. stretching Longitudinal stretching ratio: 4.0:1 SettingTemperature: 230° C.

After setting, side A of this film was corona treated, as in all of thefollowing Examples. The surface tension was 52 mN/m.

The film had the required oxygen barrier.

EXAMPLE 5

In a manner similar to that of Example 4, a three-layer film having anoverall thickness of 12 μm was produced by coextrusion. The outer layerA had a thickness of 2.0 μm, the outer layer C a thickness of 1.5 μm.

Base Layer:

100% by weight of polyethylene terephthalate having an SV of 800

Outer layer A: 70% by weight of polyethylene 2,6-naphthalate having anSV of 800, 20% by weight of polyethylene terephthalate having an SV of800 and 10% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles havingan average particle size of 2.0 μm. Outer layer C: 80% by weight ofpolyethylene terephthalate having an SV of 800 and 20% by weight ofmasterbatch made from 99.0% by weight of poly- ethylene terephthalateand 1.0% by weight of silica particles, 50% of which had an averageparticle size of 2.5 μm and 50% of which had an average particle size of1.0 μm.

The process conditions for all layers were as in Example 4.

EXAMPLE 6

A coextruded film having the recipe of Example 5, where outer layer Awas 2.0 μm thick and had the following make-up:

90% by weight of polyethylene 2,6-napthalate having an SV of 800 and 10%by weight of masterbatch made from 99.0% by weight of poly- ethyleneterephthalate and 1.0% by weight of silica particles having an averageparticle size of 1.0 μm,was produced under the process conditions of Example 4.

EXAMPLE 7

A coextruded film having the recipe of Example 5, where outer layer Awas 2.5 μm thick and had the following make-up:

90% by weight of polyethylene 2,6-naphthalate having an SV of 800 and10% by weight of masterbatch made from 99.0% by weight of polyethylene2,6-naphthalate having an SV of 800 and 1.0% by weight of silicaparticles having an average particle size of 1.0 μm,was produced under the process conditions of Example 4, but thetemperatures of longitudinal and transverse stretching were now raisedby about 10° C.

EXAMPLE 8

A three-layer coextruded film having a base layer and two outer layerswas produced in a manner similar to that of Example 5. The overallthickness of the film was 12 μm. Outer layer A had a thickness of 3 μm,and outer layer C of 1.5 μm.

Base Layer:

100% by weight of polyethylene terephthalate having an SV of 800

Outer Layer A:

100% by weight of polyethylene 2,6-naphthalate having an SV of 800

Outer layer C: 80% by weight of polyethylene terephthalate having an SVof 800 and 20% by weight of masterbatch made from 99.0% by weight ofpoly- ethylene terephthalate and 1.0% by weight of silica particles, 50%of which had an average particle size of 2.5 μm and 50% of which had anaverage particle size of 1.0 μm.

The process conditions for all layers were as given in Example 7.

EXAMPLE 9

A coextruded film was produced in a manner similar to that of Example 4,but the copolymer for the outer layers was now prepared as in Example 2.In other respects, the conditions corresponded to those in Example 4.

EXAMPLE 10

A coextruded film was produced in a manner similar to that of Example 4,but the copolymer for the outer layers was now prepared as in Example 1.In other respects, the conditions corresponded to those in Example 4.

EXAMPLE 11

A coextruded two-layer film having a base layer and an outer layer wasproduced in a manner similar to that of Example 4. The overall thicknessof the film was 12 μm, the outer layer having a thickness of 3 μm.

Base layer: 80% by weight of polyethylene terephthalate having an SV of800 and 20% by weight of masterbatch made from 99.0% by weight of poly-ethylene terephthalate and 1.0% by weight of silica particles, 50% ofwhich had an average particle size of 2.5 μm and 50% of which had anaverage particle size of 1.0 μm. Outer layer: 60% by weight ofpolyethylene naphthalate having an SV of 800, 30% by weight ofpolyethylene terephthalate having an SV of 800 and 10% by weight ofmasterbatch made from 99.0% by weight of poly- ethylene terephthalateand 1.0% by weight of silica particles having an average particle sizeof 2.0 μm.

The process conditions for all layers were as given in Example 4.

EXAMPLE 12

A three-layer film was produced as described in Example 7, but with thesingle exception that the thickness of outer layer A was only 1.0 μm.

Comparative Example 1C

A film was produced in a manner similar to that of Example 11. For outerlayer A, however, use was made of a copolyester of 82% by weight ofethylene terephthalate and 18% by weight of ethylene isophthalate.

Comparative Example 2C

A film was produced in a manner similar to that of Example 11. For outerlayer A, use was now made of a polymer mixture made from 70% by weightof ethylene terephthalate and 30% by weight of ethylene 2,6-naphthalate.

Comparative Example 3C

A film was produced in a manner similar to that of Example 11. For outerlayer A, use was now made of a polymer mixture of 90% by weight ofethylene terephthalate and 10% by weight of ethylene 2,6-naphthalate.

Comparative Example 4C

A single-layer PET film was produced with the following layers:

80% by weight of polyethylene terephthalate having an SV of 800 and 20%by weight of masterbatch made from 99.0% by weight of poly- ethyleneterephthalate and 1.0% by weight of silica particles, 50% of which hadan average particle size of 2.5 μm and 50% of which had an averageparticle size of 1.0 μm.

Comparative Example 5C

A single-layer PEN film was produced with the following make-up:

80% by weight of polyethylene 2,6-naphthalate having an SV of 1000 and20% by weight of masterbatch made from 99.0% by weight of polyethylene2,6-naphthalate and 1.0% by weight of silica particles, 50% of which hadan average particle size of 2.5 μm and 50% of which had an averageparticle size of 1.0 μm.

The film had very good barrier properties; however, due to highproduction costs it is unsuitable for food and drinks packaging.

The compositions and properties of the films produced in Examples 4 to12 and 1C to 5C are given in Tables 1 and 2.

TABLE 1 Ethylene 2,6- Ethylene Ethylene naphthalate terephthalateisophthalate units in units in units in outer layer outer layer outerlayer A A A Example (in % by (in % by (in % by T_(g) No. weight) weight)weight) (in ° C.) 4 70 30 0 82.5 5 70 30 0 81.0 6 90 10 0 86.0 7 100 0 090.0 8 100 0 0 90.0 9 70 30 0 82.5 10  70 30 0 82.5 11  60 40 0 83.0 12 100 0 0 86.0 1C 0 82 18  72.0 2C 30 70 0 81.0 3C 10 90 0 80.5 4C 0 100 080.0 5C 100 0 0 115.0

TABLE 2 Gloss Outer (60° angle Film layer Oxygen of measure- Ex- thick-thickness Film permeability ment) ample ness A/C(A) struc- (cm³/(m³ barSide Side No. (μm) (μm) ture d)) A C Haze 4 12 2.0/2.0 ABA 70 175 1752.5 5 12 2.0/1.5 ABC 80 174 175 2.6 6 12 2.0/1.5 ABC 65 176 175 2.5 7 122.5/1.5 ABC 55 155 155 4.0 8 12 3.0/1.5 ABC 45 160 155 4.0 9 12 2.0/2.0ABA 70 175 175 2.5 10  12 2.0/2.0 ABA 70 175 175 2.5 11  12 3.0 AB 80175 178 1.5 12  12 1.0/1.0 ABC 62 160 165 3.5   1C 12 3.0 AB 102  145160 3.0   2C 12 3.0 AB 110  120 150 6.5   3C 12 3.0 AB 95 175 175 1.5  4C 12 0 A 100  175 178 4.0   5C 12 0 A 30 175 178 4.0

1. A transparent, biaxially oriented polyester film comprising: (A) abase layer, at least 80% by weight of which is composed of athermoplastic polyester; and (B) at least one outer layer having athickness of from 0.2 to 6 μm, wherein the outer layer is composed of apolymer, or of a mixture of polymers comprising: at least 40% by weightof ethylene 2,6-naphthalate units; ethylene terephthalate units, whereinthe ethylene terephthalate units are present in an amount up to 40% byweight; and optionally up to 60% by weight of units from cycloaliphaticdiols, aromatic diols, cycloaliphatic dicarboxylic acids, aromaticdicarboxylic acids, or a combination thereof, wherein the glasstransition temperature (T_(g)2 value) of the polyester film is above theT_(g)2 value of the base layer but below the T_(g)2 value of the outerlayer, and at least one film surface has a surface tension of from 35 to65 mN/m or has been provided with a functional coating of thickness from5 to 100 nm or both.
 2. A film as claimed in claim 1, wherein the outerlayer comprises at least 65% by weight of ethylene 2,6-naphthalateunits.
 3. A film as claimed in claim 1, wherein the outer layercomprises at least 70% by weight of ethylene 2,6-naphthalate units.
 4. Afilm as claimed in claim 1 or 2, which has an oxygen permeability ofless than 80 cm³/(m²bar d).
 5. A film as claimed in claim 1, which hasan oxygen permeability less than 75 cm³/(m²bar d).
 6. A film as claimedin claim 1, which has an oxygen permeability of less than 70 cm³/(m²bard).
 7. A film as claimed in claim 1, wherein the outer layer has athickness of from 0.3 to 5.5 μm.
 8. A film as claimed in claim 1,wherein the outer layer has a thickness of from 0.3 to 5.0 μm.
 9. A filmas claimed in claim 1, which has two layers and is composed of the baselayer and the outer layer.
 10. A film as claimed in claim 1, having twoouter layers, one on each side of the base layer.
 11. A film as claimedin claim 1, wherein at least one of the outer layers has been pigmented.12. A film as claimed in claim 1, wherein the surface tension isachieved by means of corona treatment.
 13. A film as claimed in claim 1,wherein the functional coating comprises one or moresubstances/compositions which are selected from the group consisting ofacrylates, ethylvinyl alcohols, PVDC, water glass, hydrophilicizedpolyesters, vinyl acetates, polyvinyl acetates, polyurethanes, thealkali metal and alkaline-earth metal salts of C₁₀-C₁₈ fatty acids,butadiene copolymers with acrylonitrile or methyl methacrylate,methacrylic acid, acrylic acid and methacrylates and acrylates.
 14. Afilm as claimed in claim 1, at least one side of which has beenmetallized or has been coated with SiO_(x) or Al_(x)O_(y).
 15. A processfor producing a transparent, biaxially oriented polyester film asclaimed in claim 1, which comprises: (A) coextruding a film from thebase layer and from the one or more outer layers; (B) orienting the filmbiaxially; (C) heat-setting the oriented film; and (D) functionalizingat least one film surface at any time after completion of thecoextrusion step.
 16. A method for packaging foodstuffs and otherconsumable items, comprising packaging said foodstuffs and otherconsumable items in a film as claimed in claim
 1. 17. A method formaking a photographic film, which comprises making said photographicfilm with a film as claimed in claim
 1. 18. A method for making agraphic film, which comprises making said graphic film with a film asclaimed in claim
 1. 19. A method for making a laminatable film, whichcomprises making said laminatable film with a film as claimed inclaim
 1. 20. A method for making a metallized film, which comprisesmaking said metallized film with a film as claimed in claim
 1. 21. Amethod for making a printable film, which comprises making saidprintable film with a film as claimed in claim
 1. 22. A transparent,biaxially oriented polyester film comprising: (A) a base layer, at least80% by weight of which is composed of a thermoplastic polyester; and (B)at least one outer layer having a thickness of from 0.2 to 6 μm, whereinthe outer layer is composed of a polymer, or of a mixture of polymerscomprising: 5 to 40% by weight of ethylene 2,6-naphthalate units; morethan 40% by weight of ethylene terephthalate units, and 0 to <55% byweight of units from cycloaliphatic diols, aromatic diols,cycloaliphatic dicarboxylic acids, aromatic dicarboxylic acids, or acombination thereof, wherein the glass transition temperature (T_(g)2value) of the polyester film is above the T_(g)2 value of the base layerbut below the T_(g)2 value of the outer layer, and at least one filmsurface has a surface tension of from 35 to 65 mN/m or has been providedwith a functional coating of thickness from 5 to 100 nm or both.